Various substances, including methemoglobin, melanin, lipid, protein, calcium, iron, copper, and manganese, are responsible for the intrinsically high signal intensity observed in intracranial lesions at T1-weighted magnetic resonance (MR) imaging. Many of these substances have physical properties that lead to other specific imaging features as well. For example, lipid-containing lesions frequently produce chemical shift artifact, and some melanin-containing lesions exhibit a combination of high signal intensity on T1-weighted images and low signal intensity on T2-weighted images. The location and extent of a region of abnormal signal hyperintensity may be helpful for identifying rare diseases such as an ectopic posterior pituitary gland near the floor of the third ventricle, bilateral involvement of the dentate and lentiform nuclei in Cockayne syndrome, and involvement of the anterior temporal lobe and cerebellum in neurocutaneous melanosis. In cases in which diagnostically specific T1-weighted imaging features are lacking, findings obtained with other MR pulse sequences and other modalities can help narrow the differential diagnosis: An elevated glutamine or glutamate level at MR spectroscopy is suggestive of hepatic encephalopathy; a popcorn ball-like appearance at T2-weighted imaging, of cavernous malformations; and hyperattenuation at computed tomography, of mineral deposition disease. In many cases, a comparison of imaging features with clinical measures enables a specific diagnosis.
© RSNA, 2012.